What is Severity Occurrence Detection and RPN Number in FMEA?

What is Severity Occurrence Detection & RPN Number in FMEA?

→ Severity Occurrence Detection and RPN Number is commonly used in FMEA.

Severity in FMEA:

 “Severity” is a ranking number associated with the most serious effect for a given failure mode, based on the criteria from a scale.
→ It assesses the impact of the failure mode (the error in the process)
→ It is a relative ranking within the scope of the specific FMEA and is determined without regard to the likelihood of occurrence or detection.
→ With 1 representing the least safety concern and 
→ 10 representing the most dangerous safety concern.
→ In most cases, processes with severity scores exceeding 8 may require a fault tree analysis, which estimates the probability of the failure mode by breaking it down into further sub-elements.
→ The severity ranking is based on a relative scale ranging from 1 to 10.
→ A “10” means the effect has a dangerously high severity leading to a hazard without warning.
→ Conversely, a ranking of “1” means the severity is extremely low.


 Severity Table in FMEA:

Severity table in FMEA

👉 Download: Severity Table

Occurrence in FMEA:

→ “Occurrence” is the fact or frequency of something happening.
→ "It assesses the chance of a failure happening".
→ The rankings are based on how frequently the cause of the failure is likely to occur.
→ With 1 representing the lowest occurrence means less chance of failure and
→ 10 representing the highest occurrence means more chance of failure.
 For example, a score of 1 may be assigned to a failure that happens once in every 5 years, while a score of 10 may be assigned to a failure that occurs once per hour, once per minute, etc.




 Occurrence Table in FMEA:

Occurrence Table in FMEA

👉 Download: Occurrence Table

Detection in FMEA:

→ "Detection assesses the chance of a failure being detected."
→ "It is Based on the chances of the failure will be detected prior to the customer finding it."
→ The Detection ranking scale, like the Severity and Occurrence scales, is on a relative scale from 1 to 10.
→ With 1 representing the highest chance of detection and 
→ 10 representing the lowest chance of detection this basically means that there are no controls in place to prevent or detect.



➤ Detection Table in FMEA:

Detection Table in FMEA

👉 Download: Detection Table

RPN Number (Risk Priority Number) in FMEA: 

→ RPN = Risk priority number = Severity X Occurrence X Detection.
→ The RPN is the Risk Priority Number.
→ The RPN gives us a relative risk ranking.
→ Higher the RPN, the higher the potential risk.
→ The RPN is calculated by multiplying the three rankings together.
→ Multiply the Severity ranking times, Occurrence ranking times and Detection ranking.
→ Calculate the RPN for each failure mode and effect.
→ The RPN gives us an excellent tool to prioritize focused improvement efforts.


➥ Example:

→ If the severity score is 5, the occurrence score is 4, and detection is 3, then the RPN would be 60
→ By Rule of Thumb, high RPN value requires an action.
→ The RPN values range from 1 (absolute best) to 1000 (absolute worst)